The present invention relates to a differential apparatus capable of achieving difference in rotation between right and left driving wheels or between front and rear drive shafts of automobiles, and capable of increasing a driving force by generating a differential-limiting torque when one of driving wheels or drive shafts is rotating idle, and a method for producing such a differential apparatus, particularly to a differential apparatus having good durability and a stable differential-limiting torque and a method for producing such a differential apparatus.
A differential apparatus of an automobile is an apparatus enabling difference in rotation between right and left driving wheels or between front and rear driving wheels when the automobile turns a curve (in the case of a four-wheel driving vehicle). Widely used as a differential apparatus having such a function for automobiles is conventionally a differential apparatus comprising a pinion gear between a pair of bevel gears connected to output shafts, thereby enabling difference in rotation between output shafts by rotating the pinion gear at the time of differential rotation, in a case where a rotation force is applied to a shaft of the pinion gear from outside.
However, when one of the driving wheels is trapped in snow, sand, etc. or falls in a ditch, one wheel runs idle by differential rotation, resulting in total loss of a driving force, etc., which leads to failure to escape from the trapping. In the case of turning a curve, a wheel runs idle when a load applied to wheels on the radial inside at turn extremely decreases, thereby losing a driving force for running on a curve at a high speed.
Proposed as a differential apparatus for overcoming such a problem is, for instance, a differential apparatus having a differential limitation mechanism of a clutch disc pressure fit-type. Because a clutch disc is given biasing pressure in advance to obtain a driving force even when one of the driving wheels is not in contact with a road in this type of the differential apparatus, each of the driving wheels is constrained even when no driving force is given from the engine. Accordingly, it is disadvantageous in combination with an apparatus required to have independence from the rotation of each wheel, such as an anti-lock brake system.
A differential apparatus equipped with a viscous coupling-type, torque-sensing, differential limitation mechanism has also been proposed and put into practical use. Because a viscous coupling transmits a torque by utilizing a shear resistance of a viscous liquid such as a silicone oil, etc., it can provide smooth differential limitation effects responsive to difference in rotation between driving wheels. However, because an initial resistance is given by a viscous liquid, it still suffers from the disadvantage that the driving wheels are constraining each other.
In view of such circumstances, a differential apparatus smaller than the above differential apparatuses and having a differential limitation function without containing a particular mechanism was proposed (Japanese Patent Laid-Open No. 8-170705). This differential apparatus comprises (a) a pair of disc plates fixed to driving wheels and coaxially disposed opposite to each other in an axial direction, each of which has on an opposing surface a circumferential, continuous groove winding such that a radial position varies at a constant period; (b) a ball holder disposed between a pair of disc plates and having a plurality of radially extending guide grooves circumferentially at an equal interval; (c) balls rolling along the circumferential, continuous grooves of a pair of the opposing disc plates, one ball radially reciprocally movable in each guide groove; (d) a casing containing rotatable disc plates and a stationary ball holder; and (e) thrust washers disposed in contact with the outer surfaces of the disc plates. Because balls move back and forth in a radial direction in the guide grooves while they move circumferentially in the continuous groove, a driving force from the engine is transmitted to each of the driving wheels via each disc plate, thereby enabling differential rotation when difference in rotation occurs in the driving wheels. When one of the driving wheels runs idle, a sliding friction force is generated between the outer surfaces of the disc plates and thrust washers by a thrust force generated in the disc plates connected to the driving wheels, thereby obtaining differential limitation effects.
FIG. 18 shows one example of the differential apparatus disclosed by Japanese Patent Laid-Open No. 8-170705, and FIG. 19 shows an assembly of one disc plate, a ball holder and balls. As shown in FIG. 18, the differential apparatus of Japanese Patent Laid-Open No. 8-170705 comprises a case 1 having an open end and a coaxial annual projection 11 at the other end, a case cover 2 having an open end of the same size as that of the open end of the case 1 and a coaxial annual projection 21 at the other end, a pair of disc plates 3, 4 disposed coaxially opposite to each other, a ball holder 5 disposed between a pair of disc plates 3, 4, a plurality of balls 6 rotatably held by the ball holder 5, and a pair of thrust washers 7, 7 positioned outside the disc plates 3, 4.
The annual projection 11 coaxial with the case 1 functions as a bearing supporting one disc plate 3. The ball holder 5 engages the case 1 via a plurality of engaging members 10 positioning between an outer surface of the ball holder 5 and an inner wall of the case 1. The case 1 is provided with a plurality of apertures 14 in a side wall, such that a lubricating oil can flow into the case 1. A flange 15 formed around the open end of the case 1 is provided with a large number of apertures 15a for bolts.
The case cover 1 is in the shape of a shallow dish, whose coaxial annual projection 21 functions as a bearing rotatably supporting the other disc plate 4. The side wall of the case cover 2 is provided with a plurality of apertures 24 for permitting a lubricating oil to flow thereinto. A flange formed around the open end of the case cover 2 is provided with a large number of apertures 25a at positions corresponding to those of the apertures 15a of the case 1. The case 1 is fixed to the case cover 2 by bolts (not shown) penetrating both apertures 15a, 25a. 
The other end of each disc plate 3, 4 is provided with annual projections 32, 42 for connecting the drive shaft (not shown) of the wheel. Also, an inner surface of each annual projection 32, 42 is provided with an axially extending groove 32a, 42a for fixing the drive shaft.
Thrust washers 7, 7 are disposed between the case 1 and the disc plate 3, and between the case cover 2 and the disc plate 4, in contact with outer surfaces of the disc plates 3, 4. When there is no difference in rotation between both driving wheels (not shown), or when one of the driving wheels does not run idle, a thrust force is not applied to the disc plate 3 or 4, resulting in substantially no friction force between the disc plates 3, 4 and the thrust washers 7, 7.
Because the winding continuous grooves of the disc plates 3 and 4 extend in opposite rotational directions with the same variations, only one of the disc plates 3 will be explained below. It should be noted that the same explanation is applicable to the other disc plate 4.
As shown in FIG. 19, an opposing surface of the disc plate 3 is provided with a circumferential, continuous groove winding such that a radial position varies at a constant period, and each winding continuous groove has an arcuate cross section such that balls can roll through the groove. The ball holder 5 is provided with a large number of elongated guide holes 51 each having such a width as to receive a ball 6, in which a ball 6 can move back and forth in a radial direction. A radial length of each guide hole 51 determining a range of the radial reciprocal movement of the ball 6 is equal to the difference between the radially outermost position (position farthest from the center) and the radially innermost position (position nearest the center) of the winding continuous groove 31.
FIG. 20 is an enlarged view showing one unit region of the winding continuous groove 31 of the disc plate 3, and FIG. 21 shows a radial distance R (distance between the center of the disc plate 3 and the winding continuous groove 31) at a rotation angle xcex8 in the winding continuous groove 31. The same is true in the relation among a winding continuous groove of the other disc plate 4, the balls 6 and the guide holes 51 of the ball holder 5.
The winding continuous groove 31 is constituted by a plurality of unit regions each consisting of a first guide region 31a extending to cause balls 6 to move from a radially outer position to a radially inner position of the disc plate 3, and a second guide region 31b extending to cause balls 6 to move from a radially inner position to a radially outer position of the disc plate 3. In the case of the embodiment shown in FIGS. 19-21, each unit region has a rotation angle xcex8 of 72xc2x0, with five unit regions existing in one cycle.
When one disc plate 3 is rotated with a ball holder 5 stationary, the other disc plate 4 rotates at the same speed in the opposite direction. Accordingly, when both disc plates 3, 4 are rotated relatively by 72xc2x0, namely when each disc plate 3, 4 rotates by 36xc2x0 in the opposite direction, a ball 6 passes the unit region of the winding continuous groove 31, whereby the ball 6 moves back and forth radially by one cycle in the guide hole 51 of the ball holder 5.
To investigate the movement of balls 6 in both of the winding continuous groove 31 and the guide holes 51 in further detail, reference will be made to FIG. 22 showing mainly a second guide region 31b in the unit region of the winding continuous groove 31 of the disc plate 3. When a ball 6 is present in a radially inverting range, in which a radial distance from the center of the disc plate 3 changes from xe2x80x9cincreasexe2x80x9d to xe2x80x9cdecreasexe2x80x9d or xe2x80x9cdecreasexe2x80x9d to xe2x80x9cincrease,xe2x80x9d and when another ball 6 is in a third guide region 31c, no force is transmitted between the balls 6 and the winding continuous groove 31, though a force is transmitted to a cylindrical inner wall of the winding continuous groove 31 in other regions.
In regions of the winding continuous groove 31 except for regions a, e, f, 31c, g and h, an angle at which the ball 6 is in contact with the winding continuous groove 31 varies such that a component of a force (thrust force) of the ball 6 to the disc plate 3 in an axial direction changes in proportion to a rotation torque applied to the case 1. Thus, a differential-limiting torque by friction with the thrust washers 7, 7 is proportional to a rotation torque applied to the case 1.
Though the differential apparatus disclosed by Japanese Patent Laid-Open No. 8-170705 is advantageous in that it needs a small number of parts, thereby facilitating their assembly, because differential rotation can be achieved between both disc plates due to engagement of the balls and the winding continuous groove, and because differential limitation effects can be obtained utilizing contact characteristics of the winding continuous groove with the balls. Nevertheless, tests under the conditions that a rotation angle of a unit region in the winding continuous groove of the disc plate 3, 4 is 60xc2x0 or 72xc2x0 have revealed that it is disadvantageous in durability and torque characteristics in the following points:
(1) With respect to durability, metal peeling takes place on the inner surfaces of the winding continuous grooves of the disc plates 3, 4, and the peeling always occurs in any one of the regions a, e, If, g and h shown in FIG. 21. Also, there is a tendency that the metal peeling occurs on the side above the centerline 38 in the regions a, f and h, and on the side below the centerline 38 in the regions e and g in this figure.
(2) With respect to torque characteristics, a torque on the side of the disc plates 3, 4 varies periodically, though a constant torque is applied to the ball holder 5, and its variation period is equal to a half of the period at which the ball 6 moves along the unit region (period at which a ball moves radially back and forth in one cycle in the guide hole). This variation period is constituted by some smaller variation periods.
The inventors have found the causes of the above problems by calculation of dynamics of a force between a ball and a winding continuous groove. Referring to FIG. 22, two balls 6 exist in one unit region (within the range of rotation angle of 72xc2x0), and these balls are classified into a ball 6a and a ball 6b. The ball 6a and the ball 6b move in the winding continuous groove 31 with a circumferential interval of 36xc2x0. In the embodiment shown in FIG. 22, the ball 6a is positioned slightly rightward from a center of the region e, while the ball 6b is positioned between the region g and the region h. Both of the balls 6a and 6b are in contact with the disc plate 3 in a state shown in FIG. 1.
When a rotation torque is applied to the case 1, forces Pa and Pb are applied from the ball holder 5 to the balls 6a and 6b. Because there are ten balls in total, these forces are in an unstable state that cannot be dynamically determined only from the balance of moments of forces. In the unstable state, the levels of forces and moments are determined by finding the flexure of members. This flexure occurs in a contact portion of the ball 6 and the winding continuous groove 31 and in a contact portion of the ball 6 and the ball holder 5. Accordingly, assuming that other portions are rigid, the flexure is determined by an elastic contact theory of Hertz, and the forces Pa and Pb are determined by utilizing the flexure. Once the forces Pa and Pb are determined, a force working between the ball 6 and the winding continuous groove 31 can easily be determined.
FIGS. 23 and 24 are figures for explaining how a force working between the ball 6b and the winding continuous groove 31 is determined. In FIG. 23, a force C is determined from a first contact angle xcex1 (angle between the direction of the moving ball 6a and a perpendicular line extending from a point on the inner wall of the winding continuous groove 31 with which the ball 6a is in contact). FIG. 24 is a view taken from the Z direction (from an upper position in the paper) in FIG. 23, and a force C determined from FIG. 23 and a second contact angle xcex2 (slanting angle of a force applied to the inner wall of the winding continuous groove 31) are utilized to determine forces D and E. The force D is a contact stress working between the ball 6a and the winding continuous groove 31, and the force E is a thrust force. The contact stress D is expressed by the formula: D=Axc3x97(1/cos xcex1)xc3x97(1/cos xcex2), and the thrust force E is expressed by the formula: E=Axc3x97(1/cos xcex1)xc3x97(tan xcex2). With respect to the ball 6b, each component of the force can be determined in the same way.
Utilizing the elastic contact theory of Hertz again, the contact stress D between the ball 6 and the winding continuous groove 31 can be determined. FIG. 25 shows the calculation results of contact stress generated when the ball 6 moves in the unit region (from 0xc2x0 to 72xc2x0) with a constant torque applied from the case 1. Areas in which the contact stress is particularly large are in the regions a, e and g in this order from the left. When the direction of a torque from the case 1 is reversed, a contact stress in three regions e, f and h is particularly large.
The reasons therefor will be explained taking as an example a ball arrangement shown in FIG. 22. When the ball 6a is at a position X1 shown in FIG. 22, the ball 6a is in contact with a convexly curved inner wall 35 of the winding continuous groove 31 (having a small radius of curvature R1), as shown in FIG. 26 that is a cross-sectional view taken along the line Sxe2x80x94S in FIG. 24. On the other hand, when the ball 6b is at a position X2 shown in FIG. 21, the ball 6b is in contact with a concavely curved inner wall 36 of the winding continuous groove 31 (having a large radius of curvature R2), as shown in FIG. 27 that is a cross-sectional view taken along the line Sxe2x80x94S in FIG. 24.
According to the elastic contact theory of Hertz, the contact stress is larger in a state shown in FIG. 26 than in a state shown in FIG. 27. Because the state of FIG. 26 is achieved when the ball is in contact with the region a, e, f, g or h, the contact stress is particularly large in these regions.
That is why metal peeling takes place in the regions a, e, f, g and h of the winding continuous groove 31 in a durability test.
The calculation results of the total thrust force E generated while all balls 6 move in the unit region (from 0xc2x0 to 72xc2x0) of the winding continuous groove 31 are shown in FIG. 28. The total thrust force E varies with a period corresponding to a half of the unit region (rotation angle: 36xc2x0), and this variation period is constituted by six small variation regions. The reason therefor is as follows: Though the first contact angle xcex1 is set such that the thrust force E is constant when all balls 6 exist in regions of the winding continuous groove 31 except for the regions a, e, f, g and h, the thrust force E is larger than that at the first contact angle xcex1, when balls 6 exist in any of the regions a, e, f, g and h of the winding continuous groove 31. Accordingly, when balls 6 exist in any of the regions a, e, f, g and h of the winding continuous groove 31, the thrust force E is large. Also, the reason why a torque varied in the experiment of torque characteristics is that a friction force exerted by the thrust washers 7, 7 varied due to the change of the thrust force.
With respect to the disc plates 3, 4, they are conventionally produced by cutting a steel material such as structural carbon steel or chromium-molybdenum steel by an NC lathe with a finish-working margin left, carrying out rough working and finish working of a winding continuous groove 31 by a ball end mill, and finally carburizing the winding continuous groove to form a hardened surface layer. However, when a carburizing treatment is conducted to the winding continuous groove, edge portions of the winding continuous groove have a high-carbon content, resulting in embrittlement despite of increase in hardness. As a result, when a ball is brought into contact with an edge of the winding continuous groove, cracking may take place, resulting in the deterioration of durability of the differential apparatus and unstable torque transmission. In addition, the above production method has too many steps, suffering from a problem of high production cost.
With respect to rough working and finish working of the winding continuous groove 31 by the ball end mill 16, a large cutting resistance is applied to a tip end 16a of the ball end mill 16, because a cutting speed is zero at the tip end 16a of the ball end mill 16, thereby posing a problem that it is difficult to form the shape and locus of the winding continuous groove at a high precision.
The winding continuous groove 31 may be worked by a special end mill 261 having a round tip blade 262 having a radius R as shown in FIG. 45. However, a precisely arcuate curved surface of the winding continuous groove 31 cannot be formed by a single step of cutting, thereby necessitating, after a rough working (primary working) for roughly forming an overall shape of the winding continuous groove, a finish working (secondary working) comprising several steps of cutting to gradually improve precision and surface roughness. This leads to a long working time period. Also, because the special end mill is expensive and needs regrinding, it is more expensive than usual tools. Accordingly, there is a problem of high production cost of the differential apparatus.
As is clear from FIG. 54 exaggeratingly showing a cross-sectional shape of the guide hole 51 of the ball holder 5 in its middle portion, the inner walls of the guide hole 51 are formed with curved surface portions 51d, 51dxe2x80x2 having a radius corresponding to the radius of the ball 6 plus 0.00-2.00 mm, with slight flat portions 51e, 51exe2x80x2 on both sides of the curved surface portions 51d, 51dxe2x80x2. Because there is a small clearance between the ball 6 and both curved surface portions 51d, 51dxe2x80x2 of the guide hole 51, the ball 6 is freely movable in the guide hole 51. With such a shape, the ball 6 held in each guide hole 51 of the ball holder 5 can transmit a torque between the case 1 and both disc plates 3, 4 without much deviating from a center of the guide hole even at a high-speed rotation.
Because the curved surface portions 51d, 51dxe2x80x2 in the inner walls of the guide holes 51 cannot be formed at the same time as the formation of the guide holes 51, they are conventionally formed by cutting after the formation of the guide hole walls. After forming a guide hole having a right wall by a tool such as a punch, an end mill, etc. as a primary working, a secondary working is usually carried out by cutting the right wall by a special end mill 261xe2x80x2 having an arcuate blade 262xe2x80x2 having a radius R on a side wall as shown in FIG. 55.
However, the special end mill is not only expensive but also more costly in regrinding than usual tools, fewer in the maximum number of regrinding than usual tools, suffering from a short service life. Further, because it is difficult to work 10 or so guide holes simultaneously on the ball holder 5, it suffers from a long working time, thereby needing a method of easily working the guide holes.
With respect to the guide holes 51 of the ball holder, a contact stress is generated between the balls 6 and the inner walls of the guide holes 51, because the balls 6 move radially back and forth in the guide holes 51 while the differential apparatus is operating. The contact stress is proportional to the difference in rotation speed between both disc plates 3, 4 and their driving torque, and thus the larger the difference in rotation speed and the driving torque, the larger the contact stress. The contact stress is usually 400 kg/mm2 or more, when the torque is 100 kg-m or more, and when the difference in rotation speed is 500 rpm or more. In addition, the balls 6 slidably move back and forth in the guide holes 51 at a high speed, and their sliding speed is 2000 cycles/minute or more when the difference in rotation speed between both disc plates 3, 4 is 500 rpm or more. If the balls 6 slidably move without rotation under such a high load (causing slipping), the inner walls of the guide holes 51 are likely to be damaged.
To prevent slipping, a lubricating oil is used in the differential apparatus. Thus, slipping is usually avoided even though a high contact stress is generated. However, when a high contact stress is generated in an initial fitting period in which a new differential apparatus is subjected to a fitting operation, the slipping could not be able to be completely prevented if only a lubricating oil were used, thereby being highly likely to damage the inner walls of the guide holes 51. If the inner walls of the guide holes 51 were damaged, dust generated thereby would function like grinding powder, successively damaging other parts of the guide holes 51, thereby reducing a service life of the differential apparatus.
Driving mechanisms of automobiles are standardized by each automobile manufacturer, and substantially all parts are designed under this standard. Thus, distances between driving gears connected to an engine and follower gears of the differential apparatus rotatable by the driving gears are set constant to keep compatibility within each type of automobiles in most cases. Accordingly, to increase a transmission force in the same type of automobiles, the shapes and sizes of parts of the differential apparatuses should be increased without changing the distances between the driving gears and the follower gears. Because a distance L is set in advance between the driving gears (not shown) connected to an engine and the follower gears fixed to the differential apparatus and rotated by the driving gears, it is impossible to increase the peripheral size of the differential case 1 to which the follower gears are attached, thereby making it difficult to increase an outer diameter of a ball holder 5 contained in the differential case 1. Further, when the ball holder 5 is fixed to the casing 1 by pins, enough strength would not be ensured if a larger torque transmission were sought. Also, positioning errors of the differential case 1, into which pins are inserted, and the ball holder 5 should be within about xc2x130 xcexcm, resulting in difficulty in achieving a low production cost.
In the conventional differential apparatus as shown in FIG. 18, because peripheral projections of the ball holder 5 engage an inner-peripheral groove of the differential case 1 to transmit a torque, a large load is always applied to the groove of the differential case 1 and the projections of the ball holder 5. Accordingly, the case 1 has a complicated shape for receiving the disc plates 3, 4, the ball holder 5 and the balls 6, thereby making it necessary to provide the case 1 with enough strength and wear resistance by making it of spheroidal graphite cast iron, cast steel, etc., and by heat-treating portions, to which a large load is applied, such as the above grooves, etc.
The ball holder 5 should be provided with improved strength and hardness by conducting heat treatments such as hardening, tempering, etc. after forming it integrally with peripheral projections and guide holes 51 from a sheet made of bearing steel, etc. However, the ball holder 5 not only is thin because it is inserted between the disc plates 3, 4, but also has a complicated shape having radially extending guide holes 51, thereby being susceptible to deformation by a heat treatment. Though it may be considered that a ball holder 5 formed with a cutting margin in peripheral projections, etc. is heat-treated and then subjected to cutting, its cutting is difficult because of the peripheral projections. Therefore, the ball holder should inevitably be engaged with the differential case 1 without cutting its outer periphery and peripheral projections, with such gaps with the inner surface and groove of the differential case 1 as to absorb deformation due to a heat treatment. However, this is likely to deteriorate the calmness of the differential apparatus by pulsation, thus causing decrease in the durability thereof.
With respect to spring washers, if they were dish spring-shaped washers, wearing would occur during their use in their end surfaces, the inner surfaces of the differential case 1 and the case cover 2 and the end surfaces of the disc plates 3, 4 with which the spring washer are in contact. Accordingly, their axially inward biasing pressure gradually decreases, resulting in decrease in a thrust force, and thus being likely to reduce an initial differential-limiting force. Particularly when the biasing pressure is high (for instance, at an initial torque of 5 kg-m or more), this tendency is remarkable.
With respect to the differential-limiting means, it is required that when disc plates are different in a rotation speed, a friction force is generated in contact surfaces of the casing and the disc plates, and that each constituent part undergoes little wear. In the conventional differential-limiting means using rollers, the rollers roll in a slanted state relative to the rotation axis of the casing and the disc plates, thereby generating a sliding friction force for the differential limitation. Because a friction force is determined by the inclination of rolls in this differential-limiting means, a friction coefficient does not change largely depending on the level of the rotation force and the biasing pressure. Also, because a high Hertz stress is generated on a flat opposing surface in contact with rollers, pitting occurs in the opposing surfaces of the rollers, the casing and the disc plates, resulting in the deterioration of durability.
The differential-limiting means is classified into a torque-responsive type, a speed-responsive type, and a torque/speed-responsive type that is a combination of the former two types. In the torque-responsive-type differential-limiting means, a differential limitation mechanism is determined by a torque ratio and an initial differential-limiting torque. Here, the torque ratio is a ratio of the torques transmitted to right and left wheels or front and rear wheels, and the initial differential-limiting torque is a torque given for differential rotation under no load. For instance, the distribution of a driving torque desired for four-wheel drive vehicles having differential-limiting means between front and rear wheels may be as follows:
(1) At the time of rapid, straight acceleration, the maximum acceleration performance is sought by making the slipping of the vehicle unlikely, for instance, with the distribution of a driving torque of 60-50% for front wheels and 40-50% for rear wheels.
(2) At the time of high-speed cruising, straight stability is sought, for instance, with the distribution of a driving torque of about 50% for front wheels and about 50% for rear wheels.
(3) Driving along a predetermined course without drifting is sought, for instance, with the distribution of a driving torque of 30% for front wheels and 70% for rear wheels at the time of rapid turn or rapid acceleration on a dry road, and 40% for front wheels and 60% for rear wheels at the time of rapid turn or rapid acceleration on a snow-covered road.
(4) At the time of low-speed driving with a large radius of turn on a dry road, decrease in tight braking is prevented, for instance, with the distribution of a driving torque of 0% for front wheels and 100% for rear wheels.
(5) At the time of declutching for braking, matching with an anti-lock braking system (ABS) is sought.
To meet the above demands, a mechanism for controlling an initial differential-limiting torque is necessary. Controlling an initial differential-limiting torque makes it possible to keep the difference in the number of rotation between right and left wheels or between front and rear wheels at a certain level or less, thereby achieving good speed-responsive characteristics.
Known as a differential-limiting means having a differential limitation mechanism for preventing one of the driving wheels from running idle at the time of turning at a relatively high speed or at the time of driving on a low-friction coefficient road is a differential-limiting means having a multi-plate clutch on the rear side of bevel gears such that a thrust force of the bevel gears pushes the multi-plate clutch to generate a friction force, thereby transmitting a driving force. For instance, Japanese Patent Laid-Open No. 6-328957 discloses a differential-limiting means comprising a hydraulic cylinder for pressing a friction clutch disposed between a differential case and a side gear in an abutment direction, an air-intake valve disposed in an air pipe for connecting the hydraulic cylinder and an air tank, an exhaust valve connected to the air pipe downstream of the air-intake valve and communicating with the air, and a controller into which detection signals of the numbers of rotation of four wheels, a vehicle speed and a steering angle are supplied to calculate a differential value indicating the variation of a slipping ratio, thereby controlling the air-intake valve and the exhaust valve to reduce pressure when both wheels are slipping, and to gradually increase pressure when the differential value of the slipping ratio variation is decreasing at the time of one-wheel slipping.
Japanese Patent Laid-Open No. 8-334162 discloses a differential-limiting means comprising a differential case driven to rotate, a differential gear contained in the differential case, a friction clutch for differential limitation mounted between a side gear that is an output member of the driving gear and the differential case, a ring-shaped pressure piston disposed around a rotation axis L of the side gear for giving an engaging force to the friction clutch, a fluid pump for supplying an operating fluid for operating the piston in response to the difference in a rotation speed between the side gear and the differential case, an orifice penetrating the piston for permitting the operating fluid to flow out of an operating fluid chamber of the piston, and a differential ring disposed between the piston and the friction clutch for changing a cross section area of the orifice in response to the difference in a rotation speed between the side gear and the differential case.
In the differential-limiting means disclosed by Japanese Patent Laid-Open Nos. 6-328957 and 8-334162, a differential-limiting force can be changed by changing an engaging force given to the friction clutch in response to the difference in a rotation speed between the side gear and the differential case, thereby enabling driving even at a low friction with a road and thus improving start characteristics on a low-friction road. However, it is extremely difficult to keep the friction force constant in a half-connecting state, in a power-transmitting mechanism utilizing a slide friction like a multi-plate friction clutch as in these differential-limiting means. Particularly at a low-speed rotation, there is a problem that so-called stick slip with which clutch plates generate a static friction and a dynamic friction intermittently may occur, thereby making the differential-limiting force unstable. Also, when the friction force is made unstable by the stick slip, noises and vibration are generated, adversely affecting driving performance.
On the other hand, the differential-limiting means disclosed by Japanese Patent Laid-Open No. 8-170705 can provide stable differential limitation effects responsive to the sensed torque, without necessitating a special mechanism for obtaining differential limitation effects. In addition, it is extremely small and can be produced at a low cost, and its differential limitation effects can freely be set depending on its applications. Accordingly, this differential-limiting means is advantageously usable for every application. This differential-limiting means is further characterized in that a torque ratio and an initial differential-limiting torque can be selected under wider conditions than those of the other differential-limiting means, whereby optimum conditions adapted for a particular vehicle can be set. However, the optimum conditions are not necessarily reached for all driving conditions of vehicles. For instance, the capability of generating enough differential-limiting torque is desired in the case of rapid turn, in a case where wheels on the radial inside at turn are floated, and in a case where wheels on one side are slipping on low-friction coefficient roads such as frozen roads, etc.
To improve escapability from and drivability in mud or snow-covered roads, etc., a reaction force (so-called initial differential-limiting torque) is given in advance in the axial direction of the differential apparatus in many cases. The setting of the initial differential-limiting torque is conventionally carried out, as shown in FIG. 72, by the steps of (a) measuring the inner diameter depth (X) of the differential case 1, the height (Y) of the case cover and the height (Z) of inner parts, (b) selecting the thickness (W1, W2) of the thrust washer 7 such that [Xxe2x88x92(Y+Z)] becomes equal to a predetermined value, (c) carrying out preliminary assembly, (d) checking the initial differential-limiting torque, (e) disassembling the differential apparatus to repeat the steps (b)-(d) when the initial differential-limiting torque is NG, and (f) completing the assembly of the differential apparatus when the initial differential-limiting torque is OK.
A rotation force due to a driving force from outside is transmitted from the ball holder 5 to the balls 6 and to the disc plates 3, 4. Reaction force-supporting surfaces of the differential case 1 and the case cover 2 are subjected to a force functioning to expand them outwardly in an axial direction. Flanges of the differential case 1 and the case cover 2 screwed to each other together with gears (not shown) are relatively thin in portions to which follower gears are fixed. As a result, the differential case 1 and the case cover 2 are likely to suffer from large deformation in an axial direction. If the differential case 1 or the case cover 2 were not enough rigid to bear a force in an axial direction, the effects of differential rotation and differential limitation would be reduced. Also, the balls 6 would largely deviate from a center of the winding continuous groove 31 of the disc plate 3, resulting in decrease in the durability of the winding continuous groove 31 and balls 6.
If the differential case or the case cover did not have improved rigidity, no assurance would be achieved in the setting of an initial differential-limiting torque, the setting and change of a differential-limiting torque at the time of reassembling or after use. Further, the method of FIG. 72 is disadvantageous in that a large number of steps are needed to set an initial differential-limiting torque.
Accordingly, an object of the present invention is to provide a differential apparatus of the same type as disclosed by Japanese Patent Laid-Open No. 8-170705, having improved durability by reducing the wear of winding continuous grooves, together with less variable torque transmission and stable differential-limiting torque.
Another object of the present invention is to provide a differential apparatus free from tipping, etc. on edges of winding continuous grooves due to contact with balls, and a method for producing such a differential apparatus.
A further object of the present invention is to provide a differential apparatus having winding continuous grooves of disc plates with high-precision shape and locus, thereby exhibiting stable differential limitation effects for a long period of time.
A still further object of the present invention is to provide a method for producing the inner walls of winding continuous grooves on opposing surfaces of disc plates and the inner walls of guide holes of a ball holder at high precision and at a low cost.
A still further object of the present invention is to provide a differential apparatus having improved lubrication, wear resistance and galling resistance of winding continuous grooves and guide holes of a ball holder, thereby having improved initial fitting, and a method for producing such a differential apparatus.
A still further object of the present invention is to provide a differential apparatus capable of having large torque transmission and being produced at a low cost even though there are limitations in the size of a casing.
A still further object of the present invention is to provide a differential apparatus having improved calmness and durability by ensuring that the inner walls of a differential case strongly engages the outer periphery of a ball holder.
A still further object of the present invention is to provide a differential apparatus having a structure capable of limiting differential rotation in response to torque, whereby an initial differential-limiting force can surely be set with restricted wear of constituent parts.
A still further object of the present invention is to provide a differential apparatus equipped with a differential-limiting means having not only a wide-range friction coefficient, but also differential limitation and durability that are always stable even in use for a long period of time or under severe conditions.
A still further object of the present invention is to provide a differential apparatus equipped with an extremely small differential-limiting means that can set a stable differential limitation under any driving conditions of vehicles, such as rapid, straight acceleration, high-speed cruising, rapid turn or rapid acceleration on a dry road, rapid turn or rapid acceleration on a snow-covered road, low-speed driving with a large radius of turn on a dry road, declutching for braking, etc.
A still further object of the present invention is to provide an easy-to-assemble differential apparatus in which differential limitation can be set stably, and an initial differential-limiting torque can be obtained surely and easily.
As a result of intense research in view of the above objects, the inventors have made the following discoveries:
(1) To provide the differential apparatus with further improved durability and stable differential-limiting torque characteristics while reducing the variation of torque transmission, a contact stress should be reduced in inflected regions of the winding continuous groove.
(2) When the inner wall of the winding continuous groove is subjected to plastic working, it has a flowed structure, thereby enabling stable torque transmission while preventing cracking on its edges.
(3) By chamfering the edges of the winding continuous groove, cracking and pitting can be prevented on the edges, thereby enabling stable torque transmission.
(4) When a small groove is formed at a bottom of the winding continuous groove of the disc plate, a tip blade of a ball end mill at a zero cutting speed is not brought into contact with the bottom of the winding continuous groove during the rough- or finish-working with the ball end mill, thereby achieving good cutting of the winding continuous groove.
(5) By using a disc plate die formed with a winding continuous groove, a ball holder and rolling-forming balls, disposing each ball in one end of each guide hole of the ball holder, sandwiching the ball holder with the disc plate die and a disc plate precursor preliminarily formed with a winding continuous groove, and rotating the ball holder while keeping the disc plate die stationary and the disc plate precursor freely rotatable, thereby forcing the rolling-forming balls to move along the winding continuous groove, a concavely curved surface can be rolling-formed at a high precision on the winding continuous groove at a low cost by the pressure of the balls.
(6) By using a pair of rolling-forming discs each having guide grooves engageable with balls on a grooved surface, sandwiching a ball holder having guide holes formed by a primary working by a punch, an end mill, etc. with a pair of the rolling-forming discs from both sides with the rolling-forming balls disposed in the guide holes of the ball holder, relatively rotating both rolling-forming discs to force the rolling-forming balls to move in the guide holes, curved surface portions can be rolling-formed on the inner walls of the guide holes by the pressure of the balls.
(7) By forming a chemical treatment coating layer or further a solid lubricating layer based on molybdenum disulfide at least on the inner surfaces of the guide holes of the ball holder, the ball holder is provided with excellent wear resistance and galling resistance, and thus resistant to mar with only the chemical treatment coating layer and/or the solid lubricating layer peeling even though slipping occurs particularly in an initial fitting period.
(8) By integrally forming a ball holder such that it has large-radius portions outside the guide holes and small-radius portions at least partially between the adjacent guide holes, and causing the large-radius portions of the ball holder to engage recesses formed on the inner wall of the casing, sufficient torque transmission can be achieved even though there are limitations in the size of the casing. Also, with respect to the casing, it can surely conduct large torque transmission when a ball holder-engaging portions thereof are provided with hardness Hv of 400 or more from a surface to a depth of up to 1 mm.
(9) By implanting pins in a large number of radial through-holes of the differential case, forming recesses corresponding to the pins on the outer periphery of the ball holder, and causing the pins to engage the recesses to fix the ball holder to the differential case, the differential apparatus is provided with improved calmness and durability. The outer periphery of the ball holder can be subjected to cutting after a heat treatment.
(10) When the casing and the disc plates are assembled while being pressed to each other in an axial direction, biasing pressure is given to the disc plates, so that the entire differential apparatus including the differential case and the case cover functions as an elastic body like a spring washer, resulting in the generation of an initial differential-limiting force. Also, by disposing plain washers and bearings (particularly roller bearings) outside the disc plates, parts including the differential case, the case cover and the disc plates are prevented from being worn.
(11) By disposing roller bearings between the axially opposing surfaces of the casing and the disc plates, disposing sliding members or a roller-holding member for rotatably holding rollers in the roller bearings, and changing the sizes of the sliding members or the roller-holding member in an axial direction relative to the diameter of rollers, the sliding members or the roller-holding member is brought into contact with the casing or the disc plates to provide a variable friction coefficient. By selecting the sizes of the sliding members or the roller-holding member in an axial direction relative to the roller diameter, a wide range of friction coefficient is generated, resulting in stable differential limitation and thus improved durability of the differential-limiting means because of no slipping in the rollers.
(12) By providing the casing of the differential-limiting means with a pressure chamber connected to an operating fluid control system, supplying an operating fluid at a pressure corresponding to the driving conditions of the vehicle to the pressure chamber, and pressing the disc plates, the balls and inner parts such as thrust washers, etc. to the casing, stable differential limitation can be obtained under any driving conditions of vehicles. In addition, the differential-limiting means can be extremely miniaturized.
(13) By forming a female screw portion having a larger inner diameter than the outer diameter of the disc plate in the differential case in a flange root portion, forming a male screw portion corresponding to the female screw portion of the differential case in the case cover, and screwing the case cover to the differential case, both of the differential case and the case cover are provided with improved rigidity, ensuring the setting of an initial differential-limiting torque, and further making it easy to assemble the differential apparatus.
The present invention has been completed based on these findings.
Thus, the differential apparatus according to one embodiment of the present invention comprises (a) a casing rotated by a driving force from outside; (b) a pair of opposing disc plates coaxially disposed in the casing, an opposing surface of each disc plate being formed with a circumferentially continuous groove winding such that a radial position changes at a constant period; (c) a plurality of balls rolling in the opposing winding continuous grooves of both disc plates; and (d) a ball holder rotating integrally with the casing and having a plurality of radially extending guide holes, each guide hole movably receiving each ball, wherein the winding continuous groove of each disc plate circumferentially continuously has first guide regions each extending from a radially outer position to a radially inner position of each disc plate and second guide regions each extending from a radially inner position to a radially outer position, inflected regions each connecting each first guide region and each second guide region being larger in width and/or depth than the first guide regions and the second guide regions, whereby a contact stress between the balls and the winding continuous groove is decreased in the inflected regions.
It is preferable that the winding continuous groove is constituted by a plurality of circumferentially continuous unit regions, that each unit region is constituted by a first guide region extending to cause the balls to move from a radially outer position to a radially inner position of the disc plate, a second guide region extending to cause the balls to move from a radially inner position to a radially outer position of the disc plate, and a third guide region keeping the balls at a constant radial position of the disc plate, and that inflected regions are curved boundary regions existing between the first guide region and the second guide region, between the second guide region and the third guide region, between the third guide region and the second guide region, and between the second guide region and the first guide region. Any of the first guide region, the second guide region and the third guide region is preferably linear except for the boundary regions. Further, the inflected regions are preferably larger in width and/or depth than the other regions by 0.1-1%.
In a preferred embodiment of the present invention, the ball holder disposed between a pair of disc plates, which is engaged to the casing, has radially elongated guide holes for holding balls circumferentially at an equal interval, the radial length of each guide hole corresponding to the radial movement range of a ball.
In another preferred embodiment of the present invention, a differential-limiting means such as a thrust washer or a needle bearing is disposed between the outside surface of each disc plate and the inner wall of the casing. When a torque applied to one disc plate decreases, a sliding friction force is generated between the outside surface of the disc plate and the differential-limiting means by a thrust force generated by contact of the balls with the winding continuous groove, thereby generating a differential-limiting torque.
One example of the preferred differential apparatus of the present invention comprises (a) a casing having an opening on each side wall; (b) a pair of disc plates rotatably received in the casing and having connecting portions each connectable to a shaft rotatably supported by each opening of the casing, an opposing surface of each disc plate being formed with a circumferentially continuous groove winding such that a radial position changes at a constant period; (c) a plurality of balls rolling in the winding continuous grooves of a pair of disc plates; and (d) a ball holder disposed between a pair of disc plates and engaging the casing, the ball holder having radially elongated guide holes each holding one ball circumferentially at an equal interval, wherein the winding continuous groove has first guide regions each extending from a radially outer position to a radially inner position and second guide regions each extending from a radially inner position to a radially outer position, inflected regions each connecting each first guide region and each second guide region being larger in width and/or depth than the first guide regions and the second guide regions, whereby a contact stress between the balls and the winding continuous groove is decreased in the inflected regions.
In a further preferred embodiment of the present invention, the winding continuous groove is formed by plastic working, with its edges chamfered. The edges of the winding continuous groove are preferably provided with curved surfaces.
In a still further preferred embodiment of the present invention, the bottom of the winding continuous groove is provided with a small groove having a width of 0.1-0.5 as a ratio to the diameter of the ball.
In a still further preferred embodiment of the present invention, the winding continuous groove has a rolling-formed surface having a shape error of 100 xcexcm or less and a surface roughness Rz of 30 xcexcm or less. Such a winding continuous groove is preferably formed by forcing the rolling-forming balls to move while pressing them to the winding continuous groove.
Specifically, it is preferable that using a disc plate precursor having a groove with a rolling-forming margin left in advance, rolling-forming balls, a ball holder having radial guide holes for holding the rolling-forming balls, a disc plate die coaxially opposing to the disc plate precursor and having a guide groove for the rolling-forming balls, and a means for pressing the rolling-forming balls to the disc plate precursor, the disc plate die and the disc plate precursor are relatively rotated to rolling-form the winding continuous groove.
An apparatus for producing a ball holder for a differential apparatus according to a preferred embodiment of the present invention comprises (1) rolling-forming balls received in guide holes formed in advance in a ball holder with a plastic deformation margin left; (2) a pair of rolling-forming discs having on opposing surfaces thereof guide grooves for rotatably holding and guiding the rolling-forming balls; (3) a means for relatively rotating a pair of rolling-forming discs in a state that they are disposed on both sides of the ball holder for holding the rolling-forming balls in the guide holes, the rolling-forming balls being forced to move along the inner walls of the guide holes by the relative rotation of a pair of rolling-forming discs, thereby rolling-forming curved surface portions corresponding to the rolling-forming balls on the inner walls of the guide holes.
The method for producing such a ball holder for a differential apparatus comprises (a) forming guide holes in the ball holder with a plastic deformation margin left in advance; (b) sandwiching the ball holder coaxial with a pair of rolling-forming discs having guide grooves for rotatably holding and guiding the rolling-forming balls on opposing surfaces, with the rolling-forming balls held in the guide grooves of the ball holder; (c) relatively rotating a pair of rolling-forming discs to force the rolling-forming balls to move along the inner walls of the guide holes, thereby rolling-forming curved surface portions corresponding to the rolling-forming balls on the inner walls of the guide holes.
In another preferred embodiment of the present invention, at least inner surfaces of the guide holes of the ball holder are provided with (a) a chemical treatment coating layer, or (b) a chemical treatment coating layer and a solid lubricating layer based on molybdenum disulfide in this order from below. The solid lubricating layer is preferably formed by (a) forming a chemical treatment coating layer by treatment with a phosphate chemical treatment agent, and then (b) applying a coating composition based on molybdenum disulfide, followed by a drying or baking treatment under the conditions of room temperature to 300xc2x0 C. for 5-60 minutes.
In a further preferred embodiment of the present invention, large-radius portions of the ball holder in the peripheral portions near the guide holes are inserted into the recesses of the casing, so that large torque transmission can be achieved even though there are limitations in the outer diameter of the casing. When the large-radius portions of the ball holder freely engage the recesses of the casing, the ball holder and the casing need not have unnecessarily high working precision, resulting in decrease in a production cost. The engaging portions of the casing preferably have hardness Hv of 400 or more from surface to a depth of up to 1 mm, and such hardness is preferably given by a heat treatment comprising high-frequency hardening and tempering.
In a still further preferred embodiment of the present invention, the ball holder is integrally fixed to the casing by causing engaging members implanted in radial through-holes of the casing to engage with recesses formed on an outer periphery of the ball holder. The engaging members are preferably implanted in the casing from inside.
In a still further preferred embodiment of the present invention, the winding continuous groove formed on an opposing surface of each disc plate has a rolling-formed surface having a shape error of 100 xcexcm or less and a surface roughness Rz of 30 xcexcm or less. The winding continuous groove can be formed on an opposing surface of the disc plate by preparing (a) a disc plate precursor having a roughly worked winding continuous groove with a rolling-forming margin left in advance, (b) rolling-forming balls, (c) a ball holder having guide holes for holding the rolling-forming balls, (d) a disc plate die coaxially opposing to the disc plate precursor and having guide grooves for rolling-forming balls, and (e) a means for pressing the rolling-forming balls to the disc plate precursor, and rotating at least one of the disc plate die and the disc plate precursor to rolling-form grooves of the disc plate precursor with the rolling-forming balls at a high precision. The rolling-forming margin is preferably 0.02-2 mm, and the radius of each rolling-forming ball is preferably the radius of each ball used for the differential apparatus +0 to 2 mm.
In a still further preferred embodiment of the present invention, the disc plates are assembled in the casing in a manner that their outside surfaces are pressed to the casing in an axial direction, thereby providing an initial differential-limiting force. Plain washers and/or bearings are preferably disposed on the outside surfaces of a pair of disc plates. The bearings are preferably roller bearings. When the disc plates are assembled in the casing in a manner that they are pressed to each other, biasing pressure is given to the disc plates in advance. Accordingly, the overall differential apparatus functions as an elastic body like a spring washer, giving an initial differential-limiting force. With plain washers disposed on the outside surfaces of the disc plates, there is little wear on end surfaces of inner and outer diameters, thereby stably providing axially inward biasing pressure. Also, with the roller bearings, the casing and the disc plates can be prevented from being worn, thereby improving the durability of the differential apparatus.
In a further preferred embodiment of the present invention, the differential-limiting means disposed between the casing and the ball holder comprises a plurality of rollers and sliding members disposed between the rollers, the sliding members having contact resistance changeable with the casing and the disc plates depending on a rotation force. In another embodiment, the differential-limiting means comprises a plurality of rollers rolling in contact with the casing and the disc plates when the difference in rotation is generated between a pair of disc plates, and a roller-holding member for rotatably holding each roller at a predetermined interval in a circumferential direction, the roller-holding member being brought into contact with the casing and the disc plates depending on the rotation force.
In a still further preferred embodiment of the present invention, the casing is provided with a pressure chamber connected to a controlling operating fluid system, to supply the operating fluid to the pressure chamber with pressure variable depending on the driving conditions of a vehicle.
In a still further preferred embodiment of the present invention, the differential case is formed with a female screw portion having a larger inner diameter than the outer diameter of the disc plates in a flange root portion, and a male screw portion corresponding to the female screw portion of the differential case is formed in the case cover, the female screw portion of the differential case being fixed to the male screw portion of the case cover by screwing, thereby providing an initial differential-limiting torque. The initial differential-limiting torque is in a linear relation with a clamping torque by screwing. The screwing engagement portion is preferably fixed by welding, small screws or both to increase the rigidity of the casing, resulting in decrease in the deviation of balls from a center of the winding continuous groove, thereby improving the durability of the differential apparatus. Also, gaps between the casing and the disc plates in an axial direction can be set precisely.